The immune system responds to injury and threats from pathogens. Cytokines are low-molecular weight polypeptides or glycoproteins that stimulate biological responses in virtually all cell types. For example, cytokines regulate many of the pathways involved in the host inflammatory response to sepsis. Cytokines influence cell differentiation, proliferation and activation, and they can modulate both proinflammatory and anti-inflammatory responses to allow the host to react appropriately to pathogens.
Binding of a cytokine to its cell surface receptor initiates intracellular signaling cascades that transduce the extracellular signal to the nucleus, ultimately leading to changes in gene expression. The pathway involving the Janus kinase family of protein tyrosine kinases (JAKs) and Signal Transducers and Activators of Transcription (STATs) is engaged in the signaling of a wide range of cytokines. Generally, cytokine receptors do not have intrinsic tyrosine kinase activity, and thus require receptor-associated kinases to propagate a phosphorylation cascade. JAKs fulfill this function. Cytokines bind to their receptors, causing receptor dimerization, and this enables JAKs to phosphorylate each other as well as specific tyrosine motifs within the cytokine receptors. STATs that recognize these phosphotyrosine motifs are recruited to the receptor, and are then themselves activated by a JAK-dependent tyrosine phosphorylation event. Upon activation, STATs dissociate from the receptors, dimerize, and translocate to the nucleus to bind to specific DNA sites and alter transcription (Scott, M. J., C. J. Godshall, et al. (2002). “Jaks, STATs, Cytokines, and Sepsis.” Clin Diagn Lab Immunol 9(6): 1153-9).
The JAK family plays a role in the cytokine-dependent regulation of proliferation and function of cells involved in immune response. Currently, there are four known mammalian JAK family members: JAK1 (also known as Janus kinase-1), JAK2 (also known as Janus kinase-2), JAK3 (also known as Janus kinase, leukocyte; JAKL; L-JAK and Janus kinase-3) and TYK2 (also known as protein-tyrosine kinase 2). The JAK proteins range in size from 120 to 140 kDa and comprise seven conserved JAK homology (JH) domains; one of these is a functional catalytic kinase domain, and another is a pseudokinase domain potentially serving a regulatory function and/or serving as a docking site for STATs (Scott, Godshall et al. 2002, supra).
While JAK1, JAK2 and TYK2 are ubiquitously expressed, JAK3 is reported to be preferentially expressed in natural killer (NK) cells and not resting T cells, suggesting a role in lymphoid activation (Kawamura, M., D. W. McVicar, et al. (1994). “Molecular cloning of L-JAK, a Janus family protein-tyrosine kinase expressed in natural killer cells and activated leukocytes.” Proc Natl Acad Sci USA 91(14): 6374-8).
Not only do the cytokine-stimulated immune and inflammatory responses contribute to normal host defense, they also play roles in the pathogenesis of diseases: pathologies such as severe combined immunodeficiency (SCID) arise from hypoactivity and suppression of the immune system, and a hyperactive or inappropriate immune/inflammatory response contributes to the pathology of autoimmune diseases such as rheumatoid and psoriatic arthritis, asthma and systemic lupus erythematosus, inflammatory bowel disease, multiple sclerosis, type I diabetes mellitus, myasthenia gravis, thyroiditis, immunoglobulin nephropathies, myocarditis as well as illnesses such as scleroderma and osteoarthritis (Ortmann, R. A., T. Cheng, et al. (2000). “Janus kinases and signal transducers and activators of transcription: their roles in cytokine signaling, development and immunoregulation.” Arthritis Res 2(1): 16-32). Furthermore, syndromes with a mixed presentation of autoimmune and immunodeficiency disease are quite common (Candotti, F., L. Notarangelo, et al. (2002). “Molecular aspects of primary immunodeficiencies: lessons from cytokine and other signaling pathways.” J Clin Invest 109(10): 1261-9). Thus, therapeutic agents are typically aimed at augmentation or suppression of the immune and inflammatory pathways, accordingly.
Deficiencies in expression of JAK family members are associated with disease states. Jak1−/− mice are runted at birth, fail to nurse, and die perinatally (Rodig, S. J., M. A. Meraz, et al. (1998). “Disruption of the Jak1 gene demonstrates obligatory and nonredundant roles of the Jaks in cytokine-induced biologic responses.” Cell 93(3): 373-83). Jak2−/− mouse embryos are anemic and die around day 12.5 postcoitum due to the absence of definitive erythropoiesis. JAK2-deficient fibroblasts do not respond to IFNgamma, although responses to IFNalpha/beta and IL-6 are unaffected. JAK2 functions in signal transduction of a specific group of cytokine receptors required in definitive erythropoiesis (Neubauer, H., A. Cumano, et al. (1998). Cell 93(3): 397-409; Parganas, E., D. Wang, et al. (1998). Cell 93(3): 385-95). JAK3 appears to play a role in normal development and function of B and T lymphocytes. Mutations of JAK3 are reported to be responsible for autosomal recessive severe combined immunodeficiency (SCID) in humans (Candotti, F., S. A. Oakes, et al. (1997). “Structural and functional basis for JAK3-deficient severe combined immunodeficiency.” Blood 90(10): 3996-4003).
The JAK/STAT pathway, and in particular all four members of the JAK family, are believed to play a role in the pathogenesis of the asthmatic response, chronic obstructive pulmonary disease, bronchitis, and other related inflammatory diseases of the lower respiratory tract. For instance, the inappropriate immune responses that characterize asthma are orchestrated by a subset of CD4+ T helper cells termed T helper 2 (Th2) cells. Signaling through the cytokine receptor IL-4 stimulates JAK1 and JAK3 to activate STAT6, and signaling through IL-12 stimulates activation of JAK2 and TYK2, and subsequent phosphorylation of STAT4. STAT4 and STAT6 control multiple aspects of CD4+ T helper cell differentiation (Pernis, A. B. and P. B. Rothman (2002). “JAK-STAT signaling in asthma.” J Clin Invest 109(10): 1279-83). Furthermore, TYK2-deficient mice were found to have enhanced Th2 cell-mediated allergic airway inflammation (Seto, Y., H. Nakajima, et al. (2003). “Enhanced Th2 cell-mediated allergic inflammation in Tyk2-deficient mice.” J Immunol 170(2): 1077-83). Moreover, multiple cytokines that signal through JAK kinases have been linked to inflammatory diseases or conditions of the upper respiratory tract such as those affecting the nose and sinuses (e.g. rhinitis, sinusitis) whether classically allergic reactions or not.
The JAK/STAT pathway has also been implicated to play a role in inflammatory diseases/conditions of the eye including, but not limited to, iritis, uveitis, scleritis, conjunctivitis, as well as chronic allergic responses. Therefore, inhibition of JAK kinases may have a beneficial role in the therapeutic treatment of these diseases.
The JAK/STAT pathway, and in particular, JAK3, also plays a role in cancers of the immune system. In adult T cell leukemia/lymphoma (ATLL), human CD4+ T cells acquire a transformed phenotype, an event that correlates with acquisition of constitutive phosphorylation of JAKs and STATs. Furthermore, an association between JAK3 and STAT-1, STAT-3, and STAT-5 activation and cell-cycle progression was demonstrated by both propidium iodide staining and bromodeoxyuridine incorporation in cells of four ATLL patients tested. These results imply that JAK/STAT activation is associated with replication of leukemic cells and that therapeutic approaches aimed at JAK/STAT inhibition may be considered to halt neoplastic growth (Takemoto, S., J. C. Mulloy, et al. (1997). “Proliferation of adult T cell leukemia/lymphoma cells is associated with the constitutive activation of JAK/STAT proteins.” Proc Natl Acad Sci USA 94(25): 13897-902).
Blocking signal transduction at the level of the JAK kinases holds promise for developing treatments for human cancers. Cytokines of the interleukin 6 (IL-6) family, which activate the signal transducer gp130, are major survival and growth factors for human multiple myeloma (MM) cells. The signal transduction of gp130 is believed to involve JAK1, JAK2 and Tyk2 and the downstream effectors STAT3 and the mitogen-activated protein kinase (MAPK) pathways. In IL-6-dependent MM cell lines treated with the JAK2 inhibitor tyrphostin AG490, JAK2 kinase activity and ERK2 and STAT3 phosphorylation were inhibited. Furthermore, cell proliferation was suppressed and apoptosis was induced (De Vos, J., M. Jourdan, et al. (2000). “JAK2 tyrosine kinase inhibitor tyrphostin AG490 downregulates the mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription (STAT) pathways and induces apoptosis in myeloma cells.” Br J Haematol 109(4): 823-8). However, in some cases, AG490 can induce dormancy of tumor cells and actually then protect them from death.
Activation of JAK/STAT in cancers may occur by multiple mechanisms including cytokine stimulation (e.g. IL-6 or GM-CSF) or by a reduction in the endogenous suppressors of JAK signaling such as SOCS (suppressor or cytokine signaling) or PIAS (protein inhibitor of activated STAT) (Boudny, V., and Kovarik, J., Neoplasm. 49:349-355, 2002). Importantly, activation of STAT signaling, as well as other pathways downstream of JAKs (e.g. Akt), has been correlated with poor prognosis in many cancer types (Bowman, T., et al. Oncogene 19:2474-2488, 2000). Moreover, elevated levels of circulatin cytokines that signal through JAK/STAT may adversely impact patient health as they are thought to play a causal role in cachexia and/or chronic fatigue. As such, JAK inhibition may be therapeutic for the treatment of cancer patients for reasons that extend beyond potential anti-tumor activity. The cachexia indication may gain further mechanistic support with realization that the satiety factor leptin signals through JAKs.
Pharmacological targeting of Janus kinase 3 (JAK3) has been employed successfully to control allograft rejection and graft versus host disease (GVHD). In addition to its involvement in signaling of cytokine receptors, JAK3 is also engaged in the CD40 signaling pathway of peripheral blood monocytes. During CD40-induced maturation of myeloid dendritic cells (DCs), JAK3 activity is induced, and increases in costimulatory molecule expression, IL-12 production, and potent allogeneic stimulatory capacity are observed. A rationally designed JAK3 inhibitor WHI-P-154 prevented these effects arresting the DCs at an immature level, suggesting that immunosuppressive therapies targeting the tyrosine kinase JAK3 may also affect the function of myeloid cells (Saemann, M. D., C. Diakos, et al. (2003). “Prevention of CD40-triggered dendritic cell maturation and induction of T-cell hyporeactivity by targeting of Janus kinase 3.” Am J Transplant 3(11): 1341-9). In the mouse model system, JAK3 was also shown to be an important molecular target for treatment of autoimmune insulin-dependent (type 1) diabetes mellitus. The rationally designed JAK3 inhibitor JANEX-1 exhibited potent immunomodulatory activity and delayed the onset of diabetes in the NOD mouse model of autoimmune type 1 diabetes (Cetkovic-Cvrlje, M., A. L. Dragt, et al. (2003). “Targeting JAK3 with JANEX-1 for prevention of autoimmune type 1 diabetes in NOD mice.” Clin Immunol 106(3): 213-25).
It has been suggested that inhibition of JAK2 tyrosine kinase can be beneficial for patients with myeloproliferative disorder. (Levin, et al., Cancer Cell, vol. 7, 2005: 387-397) Myeloprofiferative disorder (MPD) includes polycythemia vera (PV), essential thrombocythemia (ET), myeloid metaplasia with myelofibrosis (MMM), chronic myelogenous leukemia (CML), chronic myelomonocytic leukemia (CMML), hypereosinophilic syndrome (HES) and systemic mast cell disease (SMCD). Although the myeloproliferative disorder (such as PV, ET and MMM) are thought to be caused by acquired somatic mutation in hematopoietic progenitors, the genetic basis for these diseases has not been known. However, it has been reported that hematopoietic cells from a majority of patients with PV and a significant number of patients with ET and MMM possess a recurrent somatic activing mutation in the JAK2 tyrosine kinase. It has also been reported that inhibition of the JAK2V617F kinase with a small molecule inhibitor leads to inhibition of proliferation of hematopoietic cells, suggesting that the JAK2 tyrosine kinase is a potential target for pharmacologic inhibition in patients with PV, ET and MMM.
Inhibition of the Jak kinases is also envisioned to have therapeutic benefits in patients suffering from skin immune disorders such as psoriasis, and skin sensitization. In psoriasis vulgaris, the most common form of psoriasis, it has been generally accepted that activated T lymphocytes are important for the maintenance of the disease and its associated psoriatic plaques (Gottlieb, A. B., et al, Nat Rev Drug Disc., 4:19-34). Psoriatic plaques contain a significant immune infiltrate, including leukocytes and monocytes, as well as multiple epidermal layers with increased keratinocyte proliferation. While the initial activation of immune cells in psoriasis occurs by an ill defined mechanism, the maintenance is believed to be dependent on a number of inflammatory cytokines, in addition to various chemokines and growth factors (JCI, 113:1664-1675). Many of these, including interleukins-2, -4, -6, -7, -12, -15, -18, and -23 as well as GM-CSF and IFNg, signal through the Janus (Jak) kinases (Adv Pharmacol. 2000; 47:113-74). As such, blocking signal transduction at the level of Jak kinases may result in therapeutic benefits in patients suffering from psoriasis or other immune disorders of the skin.
It has been known that certain therapeutics can cause immune reactions such as skin rash or diarrhea in some patients. For instance, administration of some of the new targeted anti-cancer agents such as Iressa, Erbitux, and Tarceva has induced acneiform rash with some patients. Another example is that some therapeutics used topically induce skin irritation, skin rash, contact dermatitis or allergic contact sensitization. For some patients, these immune reactions may be bothersome, but for others, the immune reactions such as rash or diarrhea may result in inability to continue the treatment. Although the driving force behind these immune reactions has not been elucidated completely at the present time, these immune reactions are likely linked to immune infiltrate.
Inhibitors of Janus kinases or related kinases are widely sought and several publications report effective classes of compounds. Examples of certain JAK inhibitors are reported in U.S. Pat. No. 6,852,727, WO 2003/011285, and U.S. Pat. App. Pub. No. 2006/0106020.
Thus, new or improved agents which inhibit Janus kinases are continually needed that act as immunosuppressive agents for organ transplants, as well as agents for the prevention and treatment of autoimmune diseases (e.g., multiple sclerosis, rheumatoid arthritis, asthma, type I diabetes, inflammatory bowel disease, Crohn's disease, autoimmune thyroid disorders, Alzheimer's disease), diseases involving a hyperactive inflammatory response (e.g., eczema), allergies, cancer (e.g., prostate, leukemia, multiple myeloma), and some immune reactions (e.g., skin rash or contact dermatitis or diarrhea) caused by other therapeutics. The compounds, compositions and methods described herein are directed toward these needs and other ends.